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Thread: sticking contacts on a safety relay

  1. #1
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    sticking contacts on a safety relay

    I have a robot surrounded with a cage, the doors in the cage have magnetic safety sensors and pallet openings have light curtains. My question is I have a company rep from the palletizer wrapping company (which included the robot) and a questionable co-worker that keeps stating the safety relay in the enclosure is stuck (he claims stuck on). My position is in the PLC (Allen Bradley RS Logix 500) something is made, telling the safety circuit to activate. Theres a 11 flash code (Allen-Bradley Guardmaster) on the safety relay. Robot guys are saying based on what this co-worker is telling them that the safety relays sometimes "stick" (whatever that means). and needs replacing. I have never heard of a safety relay "sticking". Anybody ever hear of something like this? These co-workers, (not electricians) keep powering the panel down and powering it back up to clear the safety relay fault. My contention is that if something were stuck, burned, defective, whatever, it would repeat constantly. This happened once in a while, like once every day or every other day out of a 12 hour operation.

    Your thoughts?

  2. #2
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    If the safety relay clears on a power cycle it is not "stuck".
    However it may be latching into the ON position because of a short transient pulse and not releasing thereafter until power cycled. Possibly a race condition in the programming of the PLC?
    I am not familiar with the general design of a safety relay.

  3. #3
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    Quote Originally Posted by GoldDigger View Post
    If the safety relay clears on a power cycle it is not "stuck".
    \
    This is the point I was making to them, can't be stuck, no such thing as "stuck"

  4. #4
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    Quote Originally Posted by Tbrzezicki View Post
    This is the point I was making to them, can't be stuck, no such thing as "stuck"
    I would be a lot more willing to believe a faulty sensor or bad programming of the PLC rather than a relay problem.

  5. #5
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    The situation they are describing is exactly what a "safety relay" is designed to NOT allow to take place. Safety relays have what are called "force guided contacts" and monitoring contacts of the contacts. If it is really "stuck" that would make it serious fault and that fault would be, per safety rules, non-resettable because the relay would deem itself defective and be required to fail safe. Cycling power would NOT be able to reset it.

    The relay is doing exactly what it is being told to do. Something is taking place out of the proper sequence and it is staying off because it is SUPPOSED to do that. Now the tricky part is slogging through the logic and sequence of operations, then comparing that to what the operators are doing (and whether or not that's what they were trained to do) to find the problem.
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  6. #6
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    I had found one. Robot with a caged area would trip the safety circuit and not reset. It would take a lot of opening and closing of the cage doors, tapping on the relays to jostle bad contacts, guys had tried jumping things out. I believe the error would come up on the robot pendant as an E Stop but it was in the door circuit.They had the problem before and changed the safety board inside the Fanuc robot.

    Eventually I found an Omron safety relay, socket mounted on the safety board, inside the Fanuc robot control box, had burned contacts. Changed that off their spare board and it worked perfectly. I had suspected there was an improper implementation, switching transients were not suppressed or too much, burning the contacts. They would close but must have looked open in the circuit.
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  7. #7
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    sticking contacts on a safety relay

    Some very good answers that are right on the money. When you are Interfacing PLC and Robotic Movement Controls, things like this happen when Limit Switches begin to wear. As an example of what this could be; When the Robotic Cycle is completed, it Homes that closes a PLC Input to let the PLC know the Robot is out of the way and OK to start the Production Cycle.

    When the Limit Switch begins to wear after repeating the Cycle thousands of times; it needs to be adjusted but hasn’t been. The Robot is doing exactly what it is supposed to do so there are no Faults / Errors from the Robot. The PLC is operating the Production Cycle so there are no Faults / Errors there. But the Production Cycle is waiting for the Robot to get out of the way. Killing the power and restarting brings everything back to Home Position and the Production Cycle can start again.

    To Troubleshoot and repair this, insist that the Operator of the Machine not Kill and Restart the next time this happens and notify the Troubleshooter. The Troubleshooter should make note of where the Machine is or what Operation Step the Machine is in and make note of the AB PLC Input and Output Indicators. Monitor the active Ladder Logic Program and see Which Line is holding everything up. If you go to the Switch or other Source that the PLC is waiting on and actually find the reason, you know what to do.

    Assuming that the Machine has been functioning correctly in the past and there have been no Programming changes done lately to the Ladder Logic, the Ladder Logic can probably be ruled out. If there have been recent changes to the Ladder Logic, it is possible that a Logic error is at fault. The last possible error could be a Sequence of Events by the Operator could be at fault. An example would be a shortcut by stopping the Production Cycle to do something and restarting without Homing.

    I agree with the others, the Safety Switch is not ‘Stuck’ it is still being held by the PLC and waiting for something to happen.

    JimO

  8. #8
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    Quote Originally Posted by jimo144 View Post
    Some very good answers that are right on the money. When you are Interfacing PLC and Robotic Movement Controls, things like this happen when Limit Switches begin to wear. As an example of what this could be; When the Robotic Cycle is completed, it Homes that closes a PLC Input to let the PLC know the Robot is out of the way and OK to start the Production Cycle.

    When the Limit Switch begins to wear after repeating the Cycle thousands of times; it needs to be adjusted but hasn’t been. The Robot is doing exactly what it is supposed to do so there are no Faults / Errors from the Robot. The PLC is operating the Production Cycle so there are no Faults / Errors there. But the Production Cycle is waiting for the Robot to get out of the way. Killing the power and restarting brings everything back to Home Position and the Production Cycle can start again.

    To Troubleshoot and repair this, insist that the Operator of the Machine not Kill and Restart the next time this happens and notify the Troubleshooter. The Troubleshooter should make note of where the Machine is or what Operation Step the Machine is in and make note of the AB PLC Input and Output Indicators. Monitor the active Ladder Logic Program and see Which Line is holding everything up. If you go to the Switch or other Source that the PLC is waiting on and actually find the reason, you know what to do.

    Assuming that the Machine has been functioning correctly in the past and there have been no Programming changes done lately to the Ladder Logic, the Ladder Logic can probably be ruled out. If there have been recent changes to the Ladder Logic, it is possible that a Logic error is at fault. The last possible error could be a Sequence of Events by the Operator could be at fault. An example would be a shortcut by stopping the Production Cycle to do something and restarting without Homing.

    I agree with the others, the Safety Switch is not ‘Stuck’ it is still being held by the PLC and waiting for something to happen.

    JimO
    Or in this case the safety relay is responding to its own intermittent contact failure.

    Sent from my XT1585 using Tapatalk

  9. #9
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    Typical safety relays I've used have two contact in series, so it would have to fail both set of contacts at the same time. I have seen impedance problems (solid state output feeding a solid state device) that some times will cause the output float high enough to keep it on. I doubt that is you problem but a quick meter on the circuit will put that to bed.

  10. #10
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    possable cause came up

    Quote Originally Posted by jimo144 View Post

    I agree with the others, the Safety Switch is not ‘Stuck’ it is still being held by the PLC and waiting for something to happen.

    JimO



    I finally went over by this certain robot and watched this carefully. There's a conveyor infeed to the robot that has light curtains installed, this relay is a light curtain muting relay (Allen Bradley). It has two photo-eyes before it and two after it to mute the safety light-curtain. On the boxes going to the robot there is a silver serial n umber sticker on the side and the second last photo-eye flickers once in a while when shining against the boxes. I know just one little flicker can set off a safety relay. And I notified the guy trying to diagnose this.

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